1. Field of the Invention
The present invention relates to a semiconductor memory and, more particularly, to a nonvolatile semiconductor memory such as a flash memory.
2. Description of the Related Art
EPROMs (Erasable Programmable Read Only Memories) are nonvolatile semiconductor memories that are erasable with ultraviolet rays and are electrically programmable. Flash memories recently attracting attention are nonvolatile semiconductor memories that are electrically erasable and programmable.
A nonvolatile semiconductor memory has a sense amplifier to read data out of a selected one of the memory cells contained in the memory. While a memory cell is being read, the other memory cells receive a stress voltage to age data stored in these memory cells. For example, when data is read out of a given memory cell (MIS transistor) in a flash memory, the other memory cells connected to a bit line to which the given memory cell is connected, thereby to continuously receive a stress voltage through their respective drains.
Data is stored in a selected memory cell in an EPROM by injecting electrons into the floating gate of the selected memory cell. The EPROM must hold the stored data even after a power source is cut (i.e., discontinued, or removed). The data stored in the memory cell is read by testing whether the memory cell passes a current (data "1") or not (data "0"). If the floating gate of a memory cell has a defect of leaking electrons, the cell is unable to hold data. Even if a memory cell is provided with a sound oxide film, electrons caught in the floating gate of the memory cell sometimes escape because of production unevenness in the oxide film.
The escaping of electrons in a semiconductor memory depends on the kind and degree of defects in the memory. To screen defective semiconductor memories before shipment, a temperature acceleration test and an operation test are carried out. The temperature acceleration test leaves an objective semiconductor memory at a high temperature to accelerate the aging of data stored in memory cells of the memory. These tests, however, involve many steps and take a long time because recent semiconductor memories are of large capacity. The tests, therefore, increase the cost of the semiconductor memories.
A conventional method of screening semiconductor memories accesses every memory cell (bit) of an objective semiconductor memory, reads data stored in the memory cells, and compares the data with original data. Namely, the conventional method measures the high margin of the objective semiconductor memory before and after the temperature acceleration test and compares the measurements with each other. As the capacity of EPROMs expands year after year, this method increasingly involves many processes and takes a longer time, thereby increasing the manufacturing cost of the EPROMs.